Contacts used in a three-dimensionally stacked memory or the like are formed deeply to electrically connect a lower layer wiring and an upper layer wiring with each other via a stacked structure of word lines in some cases. Because an aspect ratio of contact holes becomes high in these cases, it becomes difficult to form a silicon dioxide film on side surfaces of the contact holes. For example, when O2 plasma oxidation is applied to an ALD (Atomic Layer Deposition) method, oxygen radicals are easily deactivated and thus it is difficult to form an oxide film having a uniform film thickness to reach deep portions of the contact holes. When plasma oxidation having enhanced oxidation power, such as ozone oxidation, is applied to the ALD method, a material of the word lines (tungsten, for example) as well as a film forming source (a precursor) is also oxidized, which leads to short-circuit between the word lines and the contacts.
Furthermore, in the ALD method, after the film forming source is caused to adhere to a semiconductor substrate and the film forming source is selectively oxidized, nitrogen (N2) purge is performed. In an atmosphere at a high temperature (a temperature equal to or higher than 700° C., for example) in which the film forming source is oxidized, the material of the word lines is also oxidized even during the N2 purge if oxygen remains in a reaction chamber. The material of the word lines sublimates during the N2 purge, which may cause whiskers. Furthermore, there are cases where a partial pressure ratio (O2/H2) of H2O/H2 changes due to a gas from the semiconductor substrates when many semiconductor substrates are to be processed in a batch reaction chamber. If the partial pressure ratio of H2O/H2 changes, an oxidation rate ratio between the film forming source and the material of the word lines changes. Also in this case, there is a risk of oxidation of the material of the word lines.